1. Field of the Invention
The present invention relates to a shadow mask type color cathode-ray tube having a light weight and an improved shock resistant characteristic.
2. Description of the Prior Art
The shadow mask is used as chrominance selective electrode of a color cathode-ray tube. The structure of the shadow mask is classified to the structures shown in FIGS. 1 and 2. In the structure of FIG. 1, it comprises an aperture (2) having many through-holes on a spherical part and basical functions as the shadow mask (1); a frame (3) for reinforcing the aperture (2); and a spring (4) for fitting the shadow mask (1) on the inner peripheral part of the color cathode-ray tube. In the structure of the shadow mask (1) shown in FIG. 1, a frame (3) having high strength is required for reinforcing the aperture (2) and accordingly, it is usually formed with an iron plate having a thickness of 1.2 to 2 mm. Thus, the weight of the shadow mask is too large. When the shadow mask is supported by springs (4) having enough strength to resist certain shock applied to the color cathode-ray tube, it is disadvantageously difficult to operate the repeat fittings and detachings of the shadow mask (1) for several times which are required for the preparation of the color cathode-ray tube. Accordingly, a light weight shadow mask has been proposed and used. The structure of the shadow mask shown in FIG. 2 has been used for a small color cathode-ray tube having a diameter of less than 6". In FIG. 2, the same reference numerals designate the parts having the same function shown in FIG. 1. In the structure of FIG. 2, a flange part (2b) extended to be substantially parallel to the spherical part from the skirt part (2a) is formed to reinforce the aperture (2) itself and the frame (3) has a configuration of a window frame made of an iron or stainless steel plate and is fixed to the flange part (2b) of the aperture. The weight of the shadow mask (1) can be 1/2 to 1/3 of the weight of the shadow mask shown in FIG. 1 as a cathode-ray tube having a diameter of 6". The shadow mask having such structure has not enough strength of the aperture itself. The following disadvantages are found.
It is necessary to form regular electron beam throughholes on the spherical part of the aperture (2) (diameter of 0.10 to 0.25 mm in the case of round through-holes width of 0.13 to 0.20 mm in the case of rectangular through-holes) in high accuracy. The through-holes are formed by a photoetching method. It is difficult to form through-holes having a diameter less than a thickness of the spherical part. In both of round through-holes and rectangular through-holes, a diameter of through-holes at peripheral part is usually smaller than that of the central part, and a thickness of the aperture is usually less than 0.18 mm. Therefore, the strength of the skirt part (2a) is not enough high. When the support spring (4) is bent in the fitting and detaching operations of the shadow mask (1), the total weight is locally applied to the skirt part (2a) to cause a deformation of the skirt part (2a). The strain is applied to the spherical part to cause inferior characteristics as the shadow mask. When the frame (3) is fixed to the flange part (2b) of the aperture, it is usual to weld them by a plurality of spot weldings. The strain is remained after the welding because of unstable strength of the aperture (2) itself. The satisfactory function of the shadow mask is not give. Moreover, in the conventional color cathode-ray tube, certain shock is applied through the support spring (4) to the shadow mask (1). The color fringing on the color cathode-ray tube is caused by the shock.